1. Field of the Invention
This invention relates generally to the supply of electrical current to a telephone subscriber loop and more particularly relates to apparatus for reducing power dissipation in such systems and for varying the subscriber line feed circuit characteristics without varying feed resistance value.
2. Description of the Prior Art
The prior art contains a number of patents descriptive of circuitry which attempts to provide a constant line current for telephone subscriber lines. Examples of such circuitry is provided by reference to U.S. Pat. Nos. 3,035,122; 3,916,110; 4,056,691; 4,007,335; 4,106,084. Essentially, the various circuits described in these prior art patents serve to maintain a constant or an efficient line current in a telephone subscriber loop, the length of which may vary and therefore whose resistance may vary depending upon the distance from the subscriber to the central office.
The line feed function associated with a classic telephone circuit is essentially equivalent to two resistors which are designated as line feed resistors in series with the central office battery. The value of the battery is in the range of 48 to 60 volts, and the resistors typically vary between 200 to 800 ohms depending upon the particular telephone exchange. The line feed resistors are selected at the central office of a value necessary to produce the proper line current versus loop resistance characteristics as expected by the telephone instrument. As is understood, a long telephone line indicates a large resistance, while a short line has a less resistance. The line feed resistors are placed in series with the telephone line and selected according to the desired loop current characteristics. In any event, these resistors dissipate a relatively large amount of power and cause thermal problems at the central office.
As indicated, certain of the systems of the prior art attempt to circumvent the power dissipation problem by providing a constant current to a telephone line relatively independent of the length of the line. However, such prior art systems suffer from other problems in that the circuitry which is located at the central office also dissipates large amounts of power and accordingly, are not capable of properly functioning with the typical variations of length found in modern day telephone systems.
It is, therefore, an object of the present invention to provide apparatus for reducing power dissipation in a telephone subscriber line interface circuit, while providing the required loop current versus loop resistance characteristics. The apparatus to be described functions to reduce the necessary power capacity required at a central office, while providing a circuit which eliminates the necessity of changing the value of telephone line feed resistors. In this respect, the apparatus allows one to utilize feed resistors of a relatively small value, while insuring that the proper current is supplied to a telephone line.